This invention relates to plasma generation equipment, and is particularly directed to a plasma control arrangement, e.g., for terminating a plasma plating or etching process at a suitable endpoint. The invention is also concerned with controlling the plasma process based on detected operating parameters, for example, the current and voltage in the case of a DC plasma operation, or current, voltage, and phase of radio frequency (RF) electrical power that is being supplied, in the case of an RF plasma operation. The invention is further concerned with a system for adapting the control algorithm according to user-defined requirements.
In a typical RF plasma generator arrangement, a high power RF source produces an RF wave at a preset frequency, i.e., 13.56 MHz, and this is furnished along a power conduit to a plasma chamber. Because there is typically a severe impedance mismatch between the RF power source and the plasma chamber, an impedance matching network is interposed between the two. There are non-linearities in the plasma chamber, and because of these and because of losses in the line and in the impedance matching network, not all of the output power of the RF generator reaches the plasma chamber. Therefore, it is conventional to employ a probe at the power input to the plasma chamber to detect the voltage and current of the RF wave as it enters the plasma chamber. By accurately measuring the voltage and current as close to the chamber as possible, the user of the plasma process can obtain a better indication of the quality of the plasma. In the case of RF plasma, the phase angle is also measured. This in turn yields better control of the etching or deposition characteristics for a silicon wafer or other workpiece in the chamber. By monitoring these parameters, it is possible to determine the process endpoint by applying these parameters to mathematical and signal processing operators. Other observable parameters can be obtained, e.g., radiation from the plasma, values of magnitude error or phase error from the matching network, gas pressure, etc., and these may also be used for process control.
To date, development and implementation of process control algorithms for controlling plasma processes have been handled through either a system controller or an external computer, for example for detection of etch endpoint. This arrangement has the advantage of ease of algorithmic development by the system manufacturer. On the other hand, one major drawback has been slow response time for the system controller, i.e., on the order of several seconds or more. As an analog interface is also required, the system controller is prevented from executing sophisticated algorithms that may be needed for accurate process detection. For this reason, it would be preferable to operate detection and control algorithms in an intelligent sensor, rather than an attached computer, so that response times can be greatly decreased, e.g., on the order of milliseconds.
Another problem with the present-day approach is that the control algorithm is rather inflexible from the user's standpoint, and it is difficult for the user to make adjustments in the process control. As the existing process controller must be factory programmed, it is not possible for the user to make local adjustments in the algorithms. This makes rapid development, testing, storing, and recalling detection algorithms impossible in the current state of the art. Because the detection and control algorithms are highly proprietary to the end user, and often are highly system dependent, it would be preferable for the end user to develop these algorithms independently of the sensor manufacturer.
System controller slowness, and the conventional analog interface, prohibit the system controller from executing more sophisticated algorithms that are necessary for accurate process detection. The preferable implementation would require operation of detection software in an intelligent sensor, where response times can be greatly decreased from current response times, i.e., on the order of milliseconds.
Until quite recently, sensing technology was not available that was capable of intelligent and accurate monitoring of operating parameters, e.g., voltage, current, and phase angle, at the sensor position. This has been achieved in accordance with a system described in U.S. patent appln. Ser. No. 08/684,833, filed Jul. 13, 1996, and which matured into U.S. Pat. No. 5,770,922 issued on Jun. 23, 1998, having a common assignee herewith. Prior implementations of a process detection and control system have required a separate small computer to serve as the "intelligence" of the sensor. For this reason, additional hardware had to be tied permanently to the plasma equipment, and these implementations did not solve the problems of delayed algorithm development and slow response times. Also, because all calculations are handled at the sensor, the need for analog connections to the system controller is eliminated, thus reducing opportunity for imposed noise.
A code-building software package that employs graphic user/design interface has recently appeared. One example of commercial software is sold under the name Matlab Simulink Interface. In this software, graphical icons, or blocks, are manipulated using a computer mouse or similar device to drag-and-drop the icons on the monitor screen of a computer. There is a library of available blocks, which include standard blocks and custom user-designed blocks. The program uses the metaphor of a block diagram to represent a dynamic system. The blocks are copied from the libraries of blocks, and connected so as to form a system, with lines representing the routing of signals from one block to the next. The object-oriented software then builds digital signal processing codes that conform with the algorithm represented by the user-created block diagram. The blocks can represent, e.g., signal generators, filters, oscilloscopes, etc., permitting the user to simulate a desired signal processing environment and to observe its behavior. However, the Matlab Simulink system does not provide for downloading an algorithm or code from the computer into a ROM or other memory device of a process controller.
A plasma monitoring and control system that monitors current, voltage and phase angle of RF plasma power, and determines process end points based on changes in those parameters, is described in Turner et al. U.S. Pat. No. 5,576,629. This system requires a dedicated process control computer. There is no provision for user-programming the computer to effect changes in the process end point detection. This system has to rely on detection of harmonics to achieve any speed in process control. The Turner et al. detection system is also quite complex, as well as cumbersome and expensive. Moreover, there are no means for saving a user-defined algorithm.